Typical lasers are comprised of two mirrors and an active region cavity. In Vertical Cavity Surface Emitting Lasers (VCSELS), there are two mirrors, a bottom mirror (which is larger/more reflective) and a top mirror, through which the light is output. In addition there is an active region, in between the two, where the light is generated. The mirrors are typically “doped”, which means that impurities are inserted into the material to make them electrically conductive. The active region is typically “intrinsic”, meaning that it is not doped with the impurities to get significant conductivity.
Electrically pumped lasers take electrical input current and convert it to light power output. How much light a laser emits as a function of how much current is input depends upon several parameters. First is the details of the laser structure. A laser structure can either be optimized for high output power, but then it requires high input current, or it can be optimized for low current operation, but then it will not put out as much light.
There are three other main parameters related to laser operation: the threshold current, the differential quantum efficiency, and the maximum output power point. As shown in FIG. 1, which is an example characteristic curve 100 for a laser, the threshold current 102 is the minimum current at which a laser begins to “lase”. Below the threshold current 104, the device acts as a light emitting diode (LED). The differential quantum efficiency 106 is the change in output power per unit change in input current. The maximum output power point 108 is the maximum power the laser can output and is related to the input current required to reach that power output.
Other non-structural conditions that affect laser operation are: 1) the temperature of the laser, 2) the natural aging of the laser, and 3) degradation of the laser due to environmental, mechanical or electrical shocks.
Every semiconductor laser has an operational characteristic curve that describes the relationship between the input current and the laser's output power. One such representative curve is shown in FIG. 2. As shown in FIG. 2, when operating a laser, one typically sets two currents, the current corresponding to the “off” or ‘0’ logic state (Ibias) and the current corresponding to the “on” or ‘1’ logic state (Ibias+Imod), where Ibias is the bias current and Imod is the modulation current. When any one or more of the non-structural conditions change, the relationship between the output power and input current characteristic of the laser changes. This typically results in a “flattening” of the laser characteristic curve, referred to as being in a “degraded” state.
FIG. 3 is a copy of the curve of FIG. 2 juxtaposed with an example curve for the same device in a degraded state. As shown in FIG. 3, the degraded state results in the Ibias level being in the region where the laser is merely an LED. As a result, as shown in FIG. 3, the settings for Ibias and/or Imod must be changed to achieve the same or an acceptable output power performance.
Of course, over time in some cases, the laser will become so severely degraded such that even if both the bias and modulation current are changed and the original (or at least minimum required) output power still can not be reached. At that point, the laser can be said to be effectively, if not actually, dead.
To account for laser degradation and adapt to non-structural changes, typically the laser output is measured so the Ibias and/or Imod settings can regularly be adjusted to account for any such changes. In order to do so, designers try to sample the laser output power accurately, and in real time, to continually identify when a change to the Ibias and/or Imod settings must be made and adjust for it in a process analogous to feedback compensation in an electrical circuit.
One way this has been done for transmitter assemblies is shown in FIG. 4. A discrete detector 402 is incorporated into a package 404 that contains at least one laser 406. The package 404 includes connections, shown here as pins 408, that provide power, ground and external signal access to the laser(s) 406 and detector 402. The package 404 housing the laser(s) 406 and detector 402 has a semi-transparent window 412 located above the laser(s) 406 and detector 402 and spaced from them at a sufficient distance so that some of the light emitted by the laser(s) 406 will be reflected back to the detector 402. The detector 402 is used to sample the output light and, based upon the amount of light detected, help identify when the Ibias and/or Imod settings should be adjusted and/or when they can no longer be adjusted so that the laser is considered degraded beyond usability or “dead”.
While the above arrangement works, its use is typically limited to instances where there the number of lasers common to a detector is small (since the detector will be sampling an aggregate and can not discriminate among the different lasers), ideally with there being one detector per laser (and the pair being isolated from any other laser-detector pairs). The technique is not feasible however once the number of lasers increases to even a dozen or more. This is because, using the technique, the increased number of devices means added size, increased space requirements, more complexity, more connections, increased power requirements, reduced mean time between failures (MTBF) based upon an increased number of devices, etc.
Another approach is to use an intracavity photodetector. This approach is undesirable because the design of the intracavity photodetector must be part of the overall laser design, adding complexity and increasing cost.
An alternative approach is to place a detector structure, for example, a thin or semi-transparent Schottky contact (if a Schottky diode is used) or a grating (if a metal-semiconductor-metal (MSM) detector device is used) near the output of the laser so that light exiting the output of the laser (i.e. exiting through the mirror of the laser structure having the lower reflectivity relative to the other mirror) will pass by and through the detector structure as it exits the laser.
FIG. 5 illustrates, in simple fashion, a top emitting vertical cavity surface emitting laser (VCSEL) 500 using Schottky contact near the output of the laser.
The laser 500 has a top mirror 502, an active region 504, and a bottom mirror 506, with the bottom mirror 506 abutting the device substrate 508. Since this VCSEL is top emitting, light output 510 is through the top mirror 502. A Schottky contact 512, placed on the emission surface 514 of the laser 500, provides a current output proportional to the laser 500 output.
FIG. 6 illustrates, in simple fashion, a bottom emitting VCSEL 600 also using a Schottky contact near the output of the laser.
The laser 600 has a top mirror 602, an active region 604, and a bottom mirror 606, with the bottom mirror 606 abutting the device substrate 608. Since this VCSEL is bottom emitting, light output 510 is through the substrate 608. A Schottky contact 612 placed on the emission surface 614 of the laser 600 (also the surface of the substrate 608 opposite the surface on which laser mirrors 602, 606 and active region 604 reside) provides an output current proportional to the laser 600 output.
FIG. 7 illustrates, in simple fashion, a top emitting VCSEL 700 using a grating near the output of the laser.
The approach of FIG. 7 is identical to that of FIG. 5 except, instead of using a Schottky contact, a grating 702 and thin or semi-transparent MSM contact 704 are used.
FIG. 8 illustrates, in simple fashion, a bottom emitting VCSEL 800 using a grating near the output of the laser.
The approach of FIG. 8 is identical to that of FIG. 6 except, instead of using a Schottky contact, a grating 802 and thin or semi-transparent MSM contact 804 are used.
The approaches of FIG. 5 through FIG. 8, are easier to implement than an intracavity approach and provide an accurate reading of the laser output. However, each will also detrimentally affect the amount of exiting light as well as the mode quality.
There is no present way to accomplish the task of identifying when a laser output has degraded beyond a specified threshold or even that a laser is effectively dead with a semiconductor device incorporating an existing or established semiconductor laser design that does not impede or otherwise detrimentally affect the laser output.